1. Field of the Invention
This invention relates to an image forming apparatus for forming an electrostatic latent image using ions as a medium and, in particular, to an ion generator driving circuit for applying an RF high voltage to an ion generator for forming an electrostatic latent image corresponding to image information.
2. Description of the Related Art
As one of conventionally used image forming apparatuses, an ion-flow-type image forming apparatus in which an ion generator as an ion generating source for forming an electrostatic latent image outputs image information is known. An image forming apparatus of this type is disclosed in, e.g., Published Unexamined Japanese Patent Application No. 61-158472.
In this apparatus, an ion flow modulated in correspondence with an image signal to be formed is radiated in the form of dots by an ion generator having an ion flow generator on a dielectric drum rotated in a predetermined direction to form an electrostatic charge image, and the electrostatic charge image is developed by a developer to form a toner image. The toner image is transferred to and fixed on image forming paper by a pressure of a pressure transfer roller. A toner image not transferred to the image forming paper but remaining on the dielectric drum is removed by a toner remover, and the electrostatic charge on the dielectric drum is removed by a discharger, thereby preparing for formation of the next electrostatic charge image. Image forming paper is conveyed from an external paper feed tray of the image forming apparatus to a transfer section by a separation roller and paper-feed rollers. After image transfer and fixing, the paper is conveyed to an external paper exhaust tray by paper exhaust rollers.
Various types of drive circuits have been proposed for the ion generator in the ion-flow-type image forming apparatus having the above arrangement.
In the image forming apparatus of this type, in order to obtain a uniform image free from irregularities within one page of image forming paper on which an electrostatic latent image is to be formed, a uniform voltage must be applied to line electrodes of the ion generator.
In conventional image forming apparatuses, oscillators and transformers arranged in one-to-one correspondence with line electrodes are used to apply a signal of 1 MHz and 2,500 V to the line electrodes. However, due to variations in circuit components constituting the resonators and those in the transformers, it is difficult to uniformize characteristics of the respective resonators of all the line electrodes, e.g., output voltages, rise and fall characteristics, output frequency characteristics, and variation characteristics of output voltages caused by a change in load. Therefore, a variable resistor is inserted at the primary side of each transformer to adjust the output voltage, and the other characteristics are adjusted by controlling parts characteristics, thereby uniformizing the oscillator characteristics of the respective lines to a certain degree.
Although an image having a somewhat uniform density can be obtained, realization of high image quality is still prevented by the variations in outputs as described above.
In addition, in conventional apparatuses, since no semiconductor device which can be directly switched by a signal of 2,500 V and 1 MHz is available as a switching device, an oscillator which is constituted by a transistor and a transformer for both boosting and resonance and causes resonance with a line electrode capacitance is used. For this reason, the following problems are posed.
(1) It is difficult to increase the speed of an image formation output.
Although a high operating speed may be realized by increasing the frequency of an output voltage, since a circuit efficiency is decreased if the frequency is increased, this method is difficult to carry out. In order to realize a high frequency using the above resonator, the secondary-side inductance of the transformer must be decreased because the line electrode capacitance is determined. However, since the withstand voltage of the transistor is limited, a predetermined boosting ratio (a turn number ratio of the primary to the secondary) must be maintained, and therefore the number of turns of the transformer cannot be easily decreased. Therefore, since the reluctance of the core of the transformer must be increased to decrease the inductance of the transformer, the efficiency is reduced accordingly.
In addition, since the rise and fall of the output voltage respectively require a time interval of about several micro seconds, these rise and fall times must be ensured in addition to a time required for ion generation. Therefore, an ON time itself or a time interval from turning ON of the nth line to that of the (n+1)th line cannot be much reduced. Therefore, an extra time not contributing to the ion generation is required to interfere with realization of the high-speed image formation output.
(2) It is difficult to realize high image quality.
As described above, due to the variations in constituting parts of the oscillators, the output voltages, the rise and fall characteristics, the output frequency characteristics, and the output characteristics obtained when the line electrode capacitance as a load varies are different between the respective oscillators connected to the line electrodes. Therefore, the ion generation amounts of the respective lines become nonuniform to make it impossible to obtain a uniform image free from irregularities.
(3) To more or less correct the variations in circuit components described in item (2) above, variable resistors are used to perform adjustment as described above. This variable resistor is connected to the primary side of each transformer where a comparatively large current flows. Therefore, since the variable resistor generates heat parts having a high power capacity must be used. In addition, the manufacturing cost is increased.
(4) It is difficult to improve reliability.
A comparatively large current is flowed through the variable resistor to adjust the output voltage. Therefore, a large stress is applied on a slider contact as a mechanism portion of the variable resistor to degrade the reliability.
(5) Since the transformers are essential parts, a hybrid arrangement or a chip arrangement is difficult to achieve. Therefore, it is difficult to decrease the size of an apparatus.
(6) Since the output voltage easily changes in accordance with a temperature increase in transistors, an apparatus is unstable with respect to temperature. For this reason, an image density is sometimes different between the first and last portions output on image forming paper.